Parts such as the Taillight: Bracket, Licence, Cover, Screw, Lens Fitting, Gasket 620131680000 (nas) are included in the Ca50k 1983 Riva Taillight parts fiche.
The twelve Ca50k 1983 Riva Taillight component parts are all listed in this schematic.
On this parts fiche the Screw, Lens Fitting is the most specified component.